The invention relates to a helical scan device for magnetically recording and/or reading signals in signal tracks on a magnetic tape, comprising a drive shaft which is rotatable about an axis of rotation; a first means secured to the drive shaft and, for carrying at least one magnetic head which is rotatable about the axis of rotation in a circular path; and a mounting carrying a second means, provided with an external cylindrical guide surface whose axis extends parallel to the axis of rotation, for guiding the magnetic tape along the magnetic head.
Devices of the type defined in the opening paragraph are known from Austrian Patent Specification No. 345577 (herewith incorporated by reference). In accordance with that Patent Specification the first means may be a head disc carrying the magnetic head. The head disc is mounted for rotation between a stationary lower and upper drum of the second means, both drums forming the guide surface for the magnetic tape. Alternatively, the first means may be constructed as a rotatable cylindrical head drum, in which case the second means is a stationary lower drum which is coaxial with the head drum and which is spaced therefrom by a narrow gap. The lower drum may be formed with a helical guide edge over a part of its circumference for guiding the magnetic tape along the magnetic head carried by the head drum. In the above-described known devices the rotatable head disc and the head drum occupy fixed positions relative to the lower drum, so that the magnetic head which is secured to the head disc or head drum is movable relative to the guide surface in an invariable circular path.
Devices for magnetically recording and/or reading signals in signal tracks on a magnetic tape are employed in apparatuses having, among other operating modes, a play mode and a stand-by mode. In the play mode the magnetic tape is wrapped around the guide surface at a specific angle by means of a threading mechanism, and the magnetic tape is movable over the guide surface and along the rotating magnetic head for the purpose of recording or reading signals. In the stand-by mode there is usually no magnetic-tape transport along the magnetic head. It is common practice to maintain the rotation of the head disc or head drum in the stand-by mode of the apparatus, in order to avoid a waiting time, required to bring the magnetic head up to speed, when changing over from the stand-by mode to the play mode. However, it has been found that when the head disc or head drum is stationary and the magnetic head is in contact with the magnetic tape, the magnetic head may be damaged seriously as a result of the frictional forces between the magnetic tape and the stationary magnetic head.
In order to avoid additional wear to the magnetic head and, possibly, the magnetic tape in the stand-by mode of the apparatus, a number of measures have been proposed in order to ensure that the rotating magnetic head cannot enter into contact with the magnetic tape in the stand-by mode. One of these measures has been proposed in Netherlands Patent Application No. 8201573 (to which U.S. Pat. No. 4,578,725 corresponds, herewith incorporated by reference). In this patent a magnetic-tape deck is described which comprises two rotatable magnetic heads and a drum having a cylindrical surface for guiding a part of a magnetic tape which is wrapped around the part of the drum, and which further comprises a mechanism for moving the magnetic tape from the drum into a stand-by position when the tape deck is in the stand-by mode. In this stand-by position the rotating magnetic heads are not in contact with the magnetic tape.
A drawback of the measure described in U.S. Pat. No. 4,578,725 for preventing the magnetic heads and the magnetic tape from coming into contact with each other in the stand-by mode, is that it takes a comparatively long time to thread the magnetic tape again around the drum starting from the stand-by position. In cases in which frequent use is made of the stand-by feature or in which stand-by and playing mode rapidly succeed one another this is undesirable and often even impermissible. Examples of such cases are data-processing equipment and camera recorders. In particular for camera recorders it is required that a suddenly occurring event can be recorded immediately.
Another measure is known from U.S. Pat. No. 3,076,060 (herewith incorporated by reference) and is employed in an apparatus for recording and reading video signals. This apparatus comprises a head disc which is rotatable about a fixed axis and carries a magnetic head by means of a radially movable carrier arm. The carrier arm is actuated by a mechanism which comprises a solenoid and a lever and latching device having a plurality of resilient elements. In the stand-by mode the lever and latching device ensures that the carrier arm with the magnetic head is brought into and maintained in a position within the circumference of the head disc, so that the rotating magnetic head cannot contact the magnetic tape. In the playing mode, in which the carrier arm is released by energizing the solenoid, the carrier arm with the magnetic head is moved outwards and subsequently pressed against the magnetic tape under the influence of centrifugal forces and spring forces. A disadvantage of this measure is in particular the intricate construction necessary to move the magnetic head away from and towards the magnetic tape and keep it in the relevant positions. Such a construction leads to expensive and vulnerable devices for recording and/or reading signals and is therefore not attractive for use in consumer products such as video recorders.
Yet another measure is disclosed in British Patent Specification No. 1,255,727 (herewith incorporated by reference) and is employed in a device comprising a conical head support which carries the magnetic heads and which is rotatable between a stationary lower and upper drum over which a magnetic tape is guided in the playing mode. The device comprises a plurality of flexible plungers for lifting the magnetic tape off the lower drum in the stand-by mode, so that the tape is no longer in contact with the rotating magnetic heads. The plunger elements are carried by a support accommodated in the lower drum and by means of an axial displacement of the support of their free ends are movable through inclined openings in the circumferential wall of the lower drum. A disadvantage of this measure is that in the stand-by mode the magnetic tape is supported only at a few locations, so that the ends of the plungers, which have only a limited contact area with the magnetic tape, may cause such high local material stresses in the magnetic tape that the magnetic tape is damaged. Therefore, this construction demands a particularly effective tape-tension control. Moreover, the construction itself comprises a number of elements of very accurate dimensions, which obviously renders the construction expensive.